1. Field of the Invention
This invention relates to a calibration scheme for a multimetals continuous emissions monitor system (hereinafter xe2x80x9cmultimetals CEMSxe2x80x9d). More specifically, this invention relates to a calibration scheme for continuous monitoring of mercury emissions from stationary sources by plasma emission spectrometry.
2. Description of the Prior Art
Almost exclusively, analyzers or continuous emission monitors for gaseous pollutants such as carbon monoxide, nitrogen oxides, hydrogen chloride, etc., are calibrated using commercially-available gas mixtures that contain precise, certified amounts of the pollutant species in question These mixtures are readily prepared by commercial vendors and then certified by suitable analytical methods. The certification is most often xe2x80x9ctraceablexe2x80x9d to reference standards provided by the National Institutes of Standards and Technology (formerly the National Bureau of Standards). The continuous monitoring of hazardous air pollutant metals is an emerging technology that presents a number of unique technological challenges. Unlike the gaseous air pollutant described above, there are no sources for xe2x80x9cstandardxe2x80x9d gas mixtures containing xe2x80x9cknownxe2x80x9d amounts of metal pollutants. Metal elements exist primarily in the solid phase except at extremely high temperatures and therefore do not lend themselves to mixture and containment in the gaseous state. The one exception is mercury, which can exist as both a liquid and gas at room temperature. There have been some efforts in the commercial arena to prepare gaseous mixtures of mercury vapor (contained in cylinders) potentially useful in applications similar to that described in the present invention disclosure. However, those devices are most directly applicable for calibrating batch-sampling mercury analyzers exclusively dedicated to the detection of mercury. Batch-sampling mercury analyzers do not continuously measure mercury in sample gas. Rather, batch-sampling mercury analyzers accumulate a mercury sample over a period of time before measuring the sample. These calibrating batch-sampling mercury analyzers use analytical methodologies that are distinctly different from those employed by the multimetals CEMS. Thus far, the developers of this product have been able to certify the contents of the gas cylinder in terms of mercury concentration, but at present, are unable to provide certification of the mercury concentration of the delivered gas stream.
For the specific multimetals CEMS described above, a calibration method has been developed in which dry aerosols of known metal composition and concentration can be generated and used in lieu of unavailable gaseous calibration mixtures. This calibration method is briefly summarized here, but for a more detailed discussion please see Seltzer, M. D. and Meyer, G. A. Inductively Coupled Argon Plasma Continuous Emissions Monitor for Hazardous Air Pollutant Metals Environmental Science and Technology, Vol. 31, (1997), pp. 2665-2672, which is incorporated herein by reference. Aqueous solutions containing known amounts of dissolved metal salts, primarily metal nitrates, are delivered at a fixed rate of 1.5 mL/min into a COTS device known as an ultrasonic nebulizer. The ultrasonic nebulizer creates a fine, liquid aerosol from these solutions that is entrained by a carrier gas flow of 1.1 L/min through a desolvation system. The desolvation system consists of a heater that raises the temperature of the liquid aerosol carrier air mixture to 140xc2x0 C. that effectively evaporates the liquid water and produces water vapor. Following evaporation of the water from the liquid aerosol droplets, the dissolved metal salt component of each aerosol droplet coalesces into a solid metal salt particle. The metal salt particulate/water vapor/carrier air stream then passes through a thermoelectric cooler that reduces the temperature of the mixture to 4xc2x0 C. and effectively condenses and removes the water vapor component. Exiting the ultrasonic nebulizer device is a carrier air stream containing suspended metal salt particles. Through systematic characterization of the ultrasonic nebulizer""s efficiency, the output of the nebulizer, in terms of micrograms of metal per minute, is precisely known. Therefore, the process described above is extremely useful for calibration of the multi metal CEMS. The validity of the calibration scheme has been established through rigorous performance testing in which the accuracy of the multimetals CEMS was verified in comparison to an EPA-approved test method, as described in Seltzer, M. D. Performance Testing of a Multimetals Continuous Emissions Monitor, Journal of the Air and Waste Management Association, Vol. 50 (2000), pp. 1010-1016, which is incorporated herein by reference.
The method described above for generating calibration aerosols works extremely well for virtually all metals except mercury. Since this method employs aqueous solutions of dissolved metal salts as starting materials to generate dry aerosols of the metals in question, suitable metal salts must be available for all metals. The soluble salts of mercury, in particular mercuric nitrate, thermally decompose at relatively low temperatures (ca. 100xc2x0 C.) and therefore are not very stable in comparison to the salts of the other heavy metals, i.e., Pb, Cd, As, Be, Cr, etc. Also, the mercuric nitrate salt particles have a unique tendency to deposit or accumulate within the ultrasonic nebulizer and within various components of the multimetals CEMS sampling system and sampling interface. Because of their poor thermal stability, the mercuric nitrate salt particles tend to spontaneously decompose and produce elemental mercury vapor. The spurious release of the mercury vapor has the undesired effect of confounding the mercury calibration process, and later, during actual monitoring procedures, can produce xe2x80x9cghostxe2x80x9d signals or xe2x80x9cmemoryxe2x80x9d effects that can adversely affect measurement accuracy.
The use of a mercury permeation device to calibrate an apparatus designed to measure airborne mercury in combustor exhaust gases is described in Baldwin, D. P. et al. Testing of Continuous Sampling Air-ICP and Mercury Systems as Continuous Emission Monitors at the Diagnostic Instrumentation and Analysis Laboratory, Report IS-5144, Sep. 18-26, 2000. The technique described therein involves a permeation device to calibrate a dedicated mercury analyzer that measures mercury only, using absorption spectrometry. The method, system and apparatus of the present invention, described below, utilizes a permeation device to calibrate an instrument for mercury detection. However, the instrument of the present invention measures all metal elements, including mercury, using emission spectrometry, rather than absorption spectrometry.
Accordingly, a method for producing a mercury calibration stream is required that will provide precisely-known quantities of mercury in a thermally-stable form that will not persist in various components of the multimetals CEMS instrument after flow of the calibration stream is deliberately terminated.
A preferred embodiment of the present invention addresses the need for a simple and reliable means of generating and introducing a known concentration of mercury for calibration of analytical instrumentation used for monitoring the emission of mercury and other hazardous air pollutant metals from waste incinerators and other stationary sources of air pollutants. The calibration material should be in the same physical form, i.e., elemental vapor, as the anticipated pollutant emission from the subject stationary source in order that the amplitude and character of the response of the monitor or analyzer be similar for both the calibration material and actual pollutant species. Since extensive test evidence indicates that stationary sources including waste incinerators and coal-burning power plants emit mercury almost exclusively in the elemental vapor phase, a calibration material consisting of elemental mercury vapor is most appropriate for reasons expressed above.
A multimetals CEMS is designed to acquire and analyze a gaseous sample stream from the stationary source in question, and determine the concentrations of various metals that are categorized as hazardous air pollutants by the U.S. EPA and other regulatory agencies. See U.S. Pat. No. 5,596,405, Method and Apparatus for the Continuous Emissions Monitoring of Toxic Airborne Metals, issued to Seltzer et al, Jan. 21, 1997, U.S. Pat. No. 5,834,656, Sampling Interface for Continuous Monitoring of Emissions issued to Seltzer on Nov. 10, 1998, U.S. Pat. No. 5,908,566, Modified Plasma Torch Design for Introducing Sample Air into Inductively Coupled Argon Plasma issued to Seltzer on Jun. 1, 1999 and U.S. Pat. No. 5,986,757, Correction of Spectral Interferences Arising from CN Emission in Continuous Air Monitoring using Inductively Coupled Plasma Atomic Emission Spectroscopy issued to Seltzer on Nov. 16, 1999, all of which are incorporated herein by reference. During the analysis process, the analytical instrumentation that comprises the multimetals CEMS measures signal intensities from the individual metal elements present in the gaseous sample stream. In order to provide metal concentration values that correspond to these signal intensities, the signal response of the instrument must first be calibrated using xe2x80x9cstandardxe2x80x9d gaseous samples containing xe2x80x9cknownxe2x80x9d amounts of each individual metal element. By measuring a series of xe2x80x9cstandardxe2x80x9d samples that contain increasing amounts of a specific metal, and recording the corresponding signal intensities for each xe2x80x9cstandardxe2x80x9d, a linear relationship can be established between signal intensity and metal concentration. Typically, the calibration procedure involves measurement of a blank or xe2x80x9cZeroxe2x80x9d standard, and a high concentration or xe2x80x9cSpanxe2x80x9d standard. The resulting linear relationship forms the basis of quantitative analysis in which samples of xe2x80x9cunknownxe2x80x9d concentration are measured, their corresponding signal intensities recorded, and actual metal concentrations calculated.
In a preferred embodiment of the present invention, a method of calibration for detection of mercury pollutants by a continuous emissions analyzer is described. First, a mercury permeation tube is placed inside the constant temperature chamber and a constant flow rate of carrier air is introduced. Next, the mercury vapor/carrier air stream is introduced into a mixing device along with a constant flow of diluent air to achieve a total flow, which is the combination of the constant flow rates of the carrier air and the diluent air. The resulting calibration gas mixture contains a mercury concentration calculated by dividing the output rate of the mercury permeation tube by the total flow. Next, the calibration gas mixture is introduced into the multimetals CEMS or other analyzer device and the corresponding signal response is recorded. Next, calibration gas mixtures containing other xe2x80x9cknownxe2x80x9d amounts of mercury vapor can be introduced and measured in the same manner. Next, a linear relationship is established between signal intensity and mercury concentration. Finally, the calibration gas mixture can be reintroduced at a later time to confirm the stability of the original calibration.
A calibration system for detection of mercury pollutants by a continuous emissions analyzer, preferably a plasma emissions spectrometer. A constant temperature chamber houses a mercury permeation tube. The mercury permeation tube has a known output rate and is set inside the chamber to entrain mercury vapor into carrier air. The carrier air flows into the chamber at a constant rate from a mass flow controller or other means for controlling air flow. The mercury entrained carrier air flows from the chamber and into an aerosol mixer operably coupled to the chamber. In a preferred embodiment, a 3-way solenoid valve is set between chamber and the mixer. The valve may be set to direct the mercury entrained carrier air into the mixer or the valve may be set to direct the mercury entrained carrier air into a trap, such as an activated charcoal trap, to prevent the escape of pollutants into the atmosphere. The mixer is operably coupled to the analyzer. A means for controlling diluent air is operably coupled to the mixer and introduces diluent air into said mixer at a constant rate. A gaseous mixture having a calibration mercury concentration flows from the mixer into the analyzer at a constant rate. A graph having coordinates of analyzer signal intensity and mercury concentration is used to plot the calibration scheme. A first signal intensity generated by the analyzer in response to the calibration mercury concentration is used for the first plot on the graph. A second signal intensity generated by the analyzer in response to a blank having zero mercury concentration is used as the second plot on the graph. However, any known mercury concentration may be used as the source of the second plot, but zero mercury concentration is most preferred because it provides the slope intercept. A linear relationship between the analyzer signal intensity and the mercury concentration on the graph is established from the first plot and the second plot. The slope intercept and slope are used to create a mathematical relationship between the analyzer signal intensity and the mercury concentration. This enables the analyzer to be calibrated by inserting a known mercury concentration into the analyzer and adjusting the signal intensity to conform to the signal intensity calculated from the graph or mathematical relationship.
One object of a preferred embodiment of the present invention is to provide a novel calibration scheme for detection of mercury pollutants by a multimetals continuous emissions monitor.
Another object of a preferred embodiment of the present invention is to provide a calibration apparatus which employs COTS components including a mercury permeation tube and constant temperature chamber and uses these components in a unique application.
Another object of a preferred embodiment of the present invention is to provide a calibration apparatus which employs a mercury permeation tube to generate mercury vapor at a constant, known output rate to be used as a calibration standard for a specific process.
Another object of a preferred embodiment of the present invention is to provide a calibration apparatus in which the mercury vapor calibration standard is in the same physical form as the mercury pollutant to be detected.
Another object of a preferred embodiment of the present invention is to provide a calibration scheme that is completely compatible for use in conjunction with a multimetals continuous emissions monitor.